M13.2: Photo Merge commands, dialog with live preview, CLI

- engine: `merge.hdr`, `merge.panorama`, `merge.hdrPanorama` (bracket size from
  EXIF); MergeJob (Send) with progress/cancel, preview renders through the develop
  pipeline (deghost overlay, auto-crop frame); finish writes <name>-HDR/-Pano.dng
  next to the first source, imports, selects, applies Auto Settings / auto crop
- ui: Photo Merge dialog (HDR / Panorama / HDR Panorama) with live preview on a
  worker thread, background final merge with progress toast; context menu and
  Ctrl+H / Ctrl+M (Ctrl now means the physical Control key on macOS)
- cli: `merge hdr|panorama|hdr-panorama`, `synth-merge` for synthetic inputs
- merge::synth: procedural brackets (DNG) and panorama views; docs/merge.md

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -8,6 +8,7 @@ mod develop;
 mod edit;
 mod library;
 mod masks;
+mod merge;
 mod preset_files;
 mod query;
 mod xmp;
@@ -102,6 +103,7 @@ pub fn command_specs() -> &'static [CommandSpec] {
         v.extend(library::specs());
         v.extend(develop::specs());
         v.extend(masks::specs());
+        v.extend(merge::specs());
         v.extend(query::specs());
         v.extend(xmp::specs());
         v.extend(preset_files::specs());